The present invention relates to an exhaust gas purifying system for purifying the exhaust gas of an engine by providing a diesel particulate filter.
The restriction of the discharge quantity of particulate matter (hereafter referred to as PM) to be discharged from a diesel engine has been enhanced year by year together with NOx, CO, and HC. Only an improvement of an engine cannot respond to the enhancement of the legal restriction. Therefore, a technique is developed, which reduces the quantity of the PM to be discharged to the open air by collecting the quantity of the PM using a filter referred to as diesel particulate filter (hereafter referred to as DPF).
The DPF for directly collecting the PM includes a monolith honeycomb wall flow type filter made of ceramic and a fiber-type filter obtained by processing ceramic or metal into fiber. The exhaust gas purifying system provided with the above PDF is set in the middle of the exhaust passage of an engine to purify the exhaust gas generated by the engine, similar to other exhaust gas purifying systems.
The DPF has problems, that is, fuel consumption deteriorates because the exhaust pressure is raised proportionally to the collected quantity of the PM and that a melting loss occurs due to sudden combustion after excessive collection of the PM. Therefore, it is necessary to remove the collected the PM by burning it and regenerate a DPF. As DPF regenerating methods, an electric heater heating type, a burner heating type, and a back washing type are proposed.
However, when using any one of these above mentioned regenerating method, fuel consumption deteriorates because the PM is burned by receiving energy from the outside. Moreover, problems occur since the control for regeneration is difficult and the system becomes large and complex, and may require two DPF systems for alternately performing the PM collection and the PM combustion (DPF regeneration) are necessary.
To solve the above problems, a technique for regenerating a DPF without receiving energy from the outside is proposed. This technique oxidizes the PM with the exhaust heat from the engine to regenerate the DPF by using lower oxidation temperature of the PM using an oxidation catalyst. This DPF system is called a continuously regenerating type DPF system because the regeneration of the DPF is basically continuous. Moreover, theses systems are further-simplified into a single DPF system, which has the advantage of simplifying the regeneration control.
The NO2 generation type DPF system 1X is shown in FIG. 7 as an example of a system for oxidizing the PM by NO2 (nitrogen dioxide) to regenerate a DPF. In this system, an oxidation catalyst 3Aa is set to the upstream side of the common wall flow filter 3Ab. This oxidation catalyst 3Aa oxidizes NO (nitrogen monoxide) in the exhaust gas. Therefore, most NOx in the exhaust gas becomes NO2 in the downstream side of the oxidation catalyst 3Aa. By using the above NO2, the PM collected in the filter 3Ab in the downstream side is removed by oxidizing it into CO2 (carbon dioxide). The NO2 lowers the PM oxidizing temperature (DPF regenerating temperature) because it has a smaller energy barrier than that of O2. Therefore, the PM combustion continuously occurs through the heat energy in the exhaust gas without using the energy supplied from the outside.
In FIG. 7, E denotes a diesel engine, 2 denotes an exhaust passage, 4 denotes a fuel pump system, 5 denotes an electronic control box, 7 denotes a battery, 8 denotes a muffler, and 9 denotes a fuel tank, by reference symbols and reference numbers.
FIG. 8 shows a system 1Y obtained by improving the NO2 regenerating type DPF system in FIG. 7. In this improved system 1Y, a porous catalyst coat layer 31 of an oxidation catalyst 32A is applied to a porous wall surface 30 of a wall flow filter 3B. The system 1Y is set up so as to perform oxidation of NO and oxidation of the PM by NO2 generated by the oxidation of NO on the wall surface of the wall flow filter 3B. This configuration simplifies the system.
Further, FIG. 9 shows a continuous regeneration system 1Z. In this system, a porous catalyst coat layer 31 that comprises the oxidation catalyst 32A and a PM oxidation catalyst 32B such as oxide is applied to the porous wall surface 30 of a wall flow filter 3C. The porous catalyst coat layer 31 burns the PM accumulated in the filter 3C at a low temperature.
The DPF system with these catalysts realizes the continuous regeneration of the PM by using the oxidation reaction of the PM by catalysts and NO2, and thereby lowering the oxidation initiation exhaust gas temperature of the PM, compared to that of a common filter.
In these continuously regenerating type DPF systems, the temperature of the DPF is equal to the combustion temperature of the PM or higher when the exhaust gas temperature is approx. 350° C. or higher, thereby burning and removing the PM collected in the DPF and realizing the self-regeneration of the DPF. However, even if the DPF is constituted by the continuously regenerating type DPF system to lower the oxidation initiation exhaust gas temperature of the PM, the exhaust gas temperature of approx. 350° C. is still needed. Therefore, the oxidation pf the PM and the self-regeneration of the DPF do not occur when the exhaust gas temperature is lower than approx. 350° C., due to a low load operation, an idling operation, and the like.
Therefore, when the engine operation states at a low exhaust gas temperature such as the idling, the low load and the like are continued, a PM oxidation state is not realized even if the PM is accumulated, causing the increase in the exhaust pressure and the deterioration of the fuel consumption. Further, there is a risk of abnormal combustion of the DPF, due to the excessive collection.
Therefore, in these continuously regenerating type DPF systems, a necessary condition for the DPF regeneration is set by calculating the accumulated quantity of the PM in a filter in accordance with an engine operation condition and/or estimating the accumulated quantity of the PM in accordance with the pressure loss of the filter that corresponds to the accumulated quantity of the PM. Then, the DPF regeneration control, which forcibly raises the exhaust gas temperature and burns the accumulated PM for removal, is performed when the necessary condition for the DPF regeneration is satisfied.
For the above DPF regeneration control, Japanese Patent Laid-Open No. 1988-201309 discloses one of the methods for raising an exhaust gas temperature by throttling exhaust gas together with the use of an EGR valve. Moreover, Japanese Patent Laid-Open No. 1992-81513 discloses a method for controlling the valve opening of an exhaust throttling valve that is set to the downstream side of a trap filter (DPF) so as to keep an exhaust gas temperature in a predetermined regeneration temperature range while regenerating the filter.
These DPF regeneration controls are effective when an exhaust gas temperature that occupies a large portion of an engine operation region ranges between 200° C. and 350° C. However, there is a problem that a DPF cannot be forcibly regenerated in a region with the extremely low exhaust gas temperature because it is difficult to raise an exhaust gas temperature up to a temperature necessary for regeneration, even when the DPF regeneration control is performed by the common post injection and the like. The low exhaust gas temperature means the condition with the exhaust gas temperature of approx. 200° C. or lower, such as when an engine is operated at low load and low engine speed for idling and the like.
An NO2 regeneration type DPF system with an oxidation catalyst set to the upstream side has the following problems. When an exhaust gas temperature rises after a state in which the DPF cannot forcibly be regenerated is continued, the fuel accumulated in an oxidation catalyst is suddenly burned. Therefore, the temperature of the oxidation catalyst becomes high, causing deterioration of the catalyst and a melting loss. Moreover, the exhaust gas, the temperature of which is high due to this sudden combustion, flows into the DPF at the downstream side. Thereby, the PM accumulated in the DPF is suddenly burned, and the melting loss of the DPF occurs.
Moreover, the following problems occur when raising the temperature of exhaust gas by combining the exhaust throttling and the EGR of the former method disclosed in Japanese Patent Laid-Open No. 1988-201309. A period for raising an exhaust pressure becomes longer because the exhaust throttling is performed to raise an exhaust gas temperature in every forcible regeneration, thereby causing deterioration of the fuel consumption. Moreover, it the forcible regeneration of the latter method disclosed in Japanese Patent Laid-Open No. 1993-81513, a region for performing the exhaust throttling, that is, a region in which a valve is closed, is very wide, and a period for raising an exhaust pressure becomes longer, just like in the case of the former method, when performing an exhaust throttling control to realize a predetermined temperature range, thereby causing the deterioration of the fuel consumption.
Further, there is a problem that a high-temperature large-flow-rate exhaust gas, discharged at the time of an idling operation and the like, is not preferable for the surrounding environment.